Equipment India | May 2009

Guest column

Concrete Compaction

The desired strength and durability of concrete can’t be achieved unless compaction of concrete has been done well using good and suitable vibrators, writes JAGVIR GOYAL.

It’s a simple fact. However good the quality of concrete ingredients may be, however strict the control on its water- cement ratio, however sophisticated the equipment for the mixing may be, transportation and placement of concrete, desired strength and durability of concrete can’t be achieved unless the compaction of concrete has been done well and in the right manner. Despite the aggregates complying with the requirements of IS 383, the water being of potable quality and cement being fresh and strong, a dense mass of concrete is obtained only after full expulsion of air from it through compaction methods. Compaction of concrete should, therefore, never be taken lightly.

An increased use of ready mix concrete all over the country is being seen a lot, these days. Transit mixers carrying concrete can now be seen even in small towns and cities. Here also, smaller lots are supplied on only a dumping basis and the purchasers have to make arrangements for the placement and compaction of concrete. Larger batches are placed directly by the RMC supplier. In both cases, however, for best results and serviceability, proper arrangements for compaction of concrete are required.

Compaction equipment
While talking of equipment for concrete compaction, only one word comes to mind: the vibrator. Vibrators are the sole pieces of successful equipment for compaction of concrete. There were times when no mechanical machinery was available and rods and tamps were used for hand compaction of concrete. Today, we can’t even imagine that such rough methods were ever used. Mechanical compaction is mandatory. However, mere arrangement of vibrators is not enough. The choice of the right vibrator is equally important. Proper compaction of concrete is possible only if good and suitable vibrators are available for it.

Types of vibrators
The vibrators for concrete compaction are divided into two classes: internal vibrators and external vibrators.

Internal vibrators
These are also known as needle vibrators, immersion vibrators or poker vibrators. These produce vibrations directly in the concrete. The engine used to run them can be electric or pneumatically driven, petrol or diesel driven. The needles are of varying diameters between 25 mm to 155 mm. the main sizes are 25 mm, 40 mm, 50 mm, 60 mm, 75 mm, 100 mm, 110 mm, 140 mm and 155 mm. 40 mm and 60 mm are com¬monly used sizes. Petrol and diesel driven vibrators prove useful when electricity or compressed air sources are not available. These vibrators can easily be lifted by the operator and shifted to convenient locations for immersing the needle into a concrete mass.
Internal vibrators are suitable for jobs involving mass concreting such as raft foundations, pile caps or jobs where a good depth of concrete is involved. These vibrators are more common than external vibrators because of the ease in their use. As a single person can shift them from place to place, in smaller jobs like housing, roof slabs, beams, columns etc, only internal vibrators are put to use.

Internal vibrators can produce 10,000 to 20,000 vibrations per minute at amplitudes of 0.8 mm to 1.6 mm and ensures fast compaction of concrete. These vibrators consist of a motor unit and a flexible hose which is attached to the needle. The hose encloses a shaft in it which is driven by the engine. The shaft produces vibrations in the needle which transfers the same to the concrete on its immersion into the concrete mass. Vibrations are caused by eccentric weights attached to the shaft or the motor or the rotor of the vibrating part. The movement of these eccentrically placed weights causes vibrations in the poker which on transmission to concrete, results in the expulsion of entrapped air and compaction of concrete. Internal vibrators can be both of motor-in-head type as well as separate drive unit type. The length of flexible hoses varies from 1 m to 6 m but may be even longer. Flexible hoses are steel wire braided and able to resist wear. The tube length is kept as 300 mm to 500 mm. Motors are generally of 2 to 4 HP. Weight of vibrators varies from as low as 6 kg to even 50 kg. The shaft alone may weigh more than the drive unit. The frequency of pokers for the pendulum type vibrators is around 12,000 to 14,000 rpm while for the pneumatic type, it ranges from 15,000 to 21,000 rpm.

To make the vibrators user-friendly, the drive units are made doubly insulated and the flexible shafts are attachable by pressing a button instead of screwing them up.

External vibrators
External vibrators are also known as surface vibrators or form vibrators. Unlike internal vibrators, these don’t come in direct contact with concrete but compact it by transferring the vibrations through the forms or surfaces holding the concrete. Needless to mention, the forms or surfaces holding concrete in this case need to be strong enough to take the vibrations caused by external vibrations.

External vibrators are best suited for pre-cast concrete components like pre-stressed poles and other such products. Sometimes, a mesh like reinforcement is used in concrete. In such cases, external vibrators are better as it is not possible for the poker to penetrate the mesh during compaction of concrete. Certain constr¬uctions demand laying of heavy reinforce¬ments at very little bar-to-bar spacing. In such cases, form vibrators can be used. Concrete in road pavements and canal linings is also better compacted through form vibrators. The time of application of form vibrators is much more than that for needle vibrators. The form work has also to be very strong to avoid any sway or failure due to heavy vibrations produced by form vibrators. Surface vibrators lose effectiveness along the edges of floors as vibrations are not fully transmitted due to area restriction. Surface vibrators aren’t used much in India.

External vibrators have lesser frequency than internal vibrators, the range being between 1,500 to 6,000 vibrations per minute. The amplitude is around 0.4 mm to 0.5 mm. Therefore, the vibrations produced by these vibrators reach 8 to 10 inch depths of concrete. Their base pan is located on the forms for concrete. An electric or pneumatic engine produces vibrations in the pan which transmits them to the forms and in turn, forms transmit them to the concrete. For surface vibrators, 6,000 rpm can be counted as high speed, 3,000 rpm as medium and 1,500 rpm low speed. The speed of pneumatic vibrators is , however, plus 9,000 rpm and can be as high as 20,000 rpm.

Preparing the plan
While preparing the plan for comp¬action of concrete, the following points should be kept in mind:

• The vibrators must be of sufficient capacity to ensure full compaction of concrete matching with the speed of its placement. If vibrators have less capacity or are insufficient in number, loose concrete will keep accumulating at site and a situation will arise when some layers of it will get covered by subsequent layers without compaction.
• The vibrators must be of suitable type for the job in hand. Here, the spacing of reinforcement is the major deciding factor. In some situations like turbo-generator deck or other heavily loaded structures, spacing of steel is very small and many layers of large diameter steel bars are to be laid. In such situations it has to be seen that the vibrations reach every corner of the formwork and the concrete gets fully compacted.
• Accessibility of all areas to be concreted is another important factor. The vibrators chosen should be able to access all parts. Often, long columns and high rise structures allow only limited access. In such cases, the type of vibrators to be used should be decided carefully. Inaccessible areas may demand compaction of concrete through surface vibrators.

Deploying skilled operators
Needle vibrators need dynamic operators who are swift in moving vibrator needles from one point to another at split second reaction time. However, as the operators are required to handle and move the heavy needles, they need to work in pairs. While one operator operates the needle, the other should be shifting the motor unit. After a suitable gap of say, one hour, they should swap places. Whenever this arrangement is not followed, the quality of compaction of concrete may suffer.

New trends in vibrators
There have been some good innov¬ations made by the vibrator producing industry. A few of them are:

High cycle vibrators
While the common poker vibrators operate at a frequency of 50/60 Hz, high cycle vibrators operating at a frequency of 180 Hz are now available. The electric motors used in these vibrators are three phase low induction motors instead of single phase and are located within the vibrator head itself. These vibrators are good for low slump concrete used in heavy constructions. High frequency maximises their productivity. These produce around 11,000 vibrations per minute and weigh between 15 to 25 kgs.

Computer controlled vibrators
Computer controlled vibrators have a computerised arrangement to supply more power to the head as soon as it meets resistance on being placing in the concrete. This helps in maintaining the motor rpm and frequency which otherwise gets reduced on meeting resistance. These vibrators can operate even under zero slump conditions.

Frequency converters
Now, internal vibrator units with built- in electronic frequency converters have been invented by the manufacturers. These can be plugged to any wall socket and are easy to operate. While using them, it has to be seen that total amperes of the poker vibrator does not exceed the stated amperes of frequency converter.

Blade attachments
Sometimes the reinforcement is so dense that it is not possible to insert needles in between the bars. For such cases, a special blade attachment has been invented and this blade replaces the needle. It can now be inserted between the bars and compaction can be done.

The vibrator industry
Many small and big industrial units are producing concrete vibrators in India. Dynapac, Aquarius, Global Impex, Univ¬ersal, MK, Taurus are some of them. Dynapac has researched the subject well and has produced both needle type as well as external vibrators suitable for all small and big jobs. It has produced pendulum vibrators, high frequency pokers powered by single phase motors for small jobs, motor in head pokers and pneumatic vibrators. It also provides technical support to calculate the vibrator force required, numbers of vibrators required and their positioning in case of external vibrators. Experience has made the manufacturers knowledgeable on correct frequency and amplitude choice and a variety of vibrators are now available. Multiquip and Denver are producing high cycle vibrators these days. In India, distributors like Sona Sales supply high frequency concrete vibrators with elec¬tronic integrated invertors manufactured by Vibromaster, Korea.
* Jagvir Goyal is the author of several technical books, a columnist for technical journals, Deputy Chief Engineer Civil in Punjab and the recipient of the TIET Distinguished Alumni Award, 2005.


TO ACHIEVE BEST COMPACTION
• Always keep the needle vertical. Never lay it on the concrete. Never force it into the concrete. Let it travel to the bottom on its own.
• Don’t move the concrete laterally with the help of the needle.
• Never over-vibrate. Over vibration will draw a laitance layer on the surface of concrete by pulling out the cement particles. Decide the time of vibration at a particular point as per site experience. It may be somewhere between 20 to 60 seconds. Keep an eye on the rising bubbles of air. The moment these bubbles stop appearing, compaction has been done at that point. The lower the slump, the higher is the compaction time at a point.
• Never pull out the needle from the concrete in an abrupt manner. Such an action will leave a vertical hole in the concrete. Withdraw it slowly.
• Don’t stop the vibrator with the needle immersed in the concrete. If it is done, withdrawal of needle will become difficult.
• Never run the vibrator in the air. Switch it on when the needle has been vertically rested on the concrete surface.
• To avoid a cold joint, see that the needle penetrates the current layer and enters the fully compacted layer below it by three to four inches.
• Never rest the motor unit of the vibrators on the steel reinforcement laid for concreting.
• Never allow the needle to touch the formwork or steel reinforcement. Prolonged touching may cause travel of vibrations and accidents.


WHILE USING INTERNAL VIBRATORS
• Check the spacing of reinforcement and know the minimum spacing of the bars. Such should be the size of the needles that these may easily pass through the spacing of steel bars.
• The diameter of the needle should be as large as possible. It will allow better compaction of concrete.
• Assess the distance of the platform where the motor unit of the vibrator will be kept from the pouring points. Choose the length of flexible hose accordingly. Unnecessarily long hoses when not required hinder work. Therefore, choosing longest hoses is not always right.
• The more the workability or slump of concrete, the less can be the diameter of the needle.
• Electrically driven internal vibrators prove cheaper than petrol or diesel driven vibrators. Try to use electrically driven vibrators.
• Pokers or needles often demand repair. The number of damaged needles may be high if the right diameter of needles has not been chosen or operators are not skilled in using them in the right manner. Keep a good number of spare needles available at site for quick replacement where required.
• Choose high frequency vibrators produced by reputed manufacturers. See that the poker tubes and caps are made of specially hardened magnesium steel alloy.
• Look for steel reinforced hoses.
• Look for a good guarantee period as vibrators are liable to breakdown. Look for such vibrator models that require least maintenance and whose spare parts are easily available.
• Whenever pneumatic vibrators are used, on completion of work, the compressor should be first switched off and then the vibrator.

 

 

 

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